Roughened copper foils have been proposed for printed circuit board suitable to form fine-pitch circuits. Such copper foils have roughened surfaces with fine irregularities formed through oxidation treatment and then reduction treatment (hereinafter sometimes collectively referred to as oxidation-reduction treatment).
For example, Patent Document 1 (WO2014/126193) discloses a surface-treated copper foil which is provided with a roughened layer having acicular fine irregularities composed of a copper complex compound having a maximum length of 500 nm or less. Furthermore, Patent Document 2 (WO2015/040998) discloses a copper foil which has, on at least one side thereof, a roughened layer having fine irregularities, i.e., acicular protrusions having a maximum length of 500 nm or less composed of a copper composite compound and has a silane coupling agent layer on the surface of the roughened layer. These roughened copper foils disclosed in these documents can achieve high adhesion to the insulating resin substrate by the anchor effect due to the fine irregularities of the layer and also enable formation of fine-pitch circuits with high etching factors. Both the roughened layers having fine irregularities disclosed in Patent Documents 1 and 2 are formed through oxidation-reduction treatment after preliminary treatment such as alkaline degreasing. The fine irregularities formed in this way have a unique shape of acicular crystals composed of a copper composite compound. The roughened surfaces having such fine irregularities are generally finer than roughened surfaces formed by the attachment of the fine copper particles and roughened surfaces formed by etching.